1. Field of the Invention
The present invention relates to device and method for heating a fuel cell stack and to a fuel cell system having the heating device.
2. Description of the Related Art
The deposits of fossil fuels, such as coal, gas and petroleum, generally used as conventional energy sources are limited, so substitutional energy that can substitute for fossil fuels has become a great matter of social and national concern and interest in recent years. For example, the need for power generation using solar heat, tidal power and wind power instead of fossil fuels, such as coal, gas and petroleum, or the need for power generation using fuel cells is emphasized.
Of the proposed substitutional energy sources, fuel cells are designed to generate electricity using a reverse reaction of the electrolytic reaction of water. The fuel cells use a technology of converting oxygen contained in air and hydrogen contained in hydrocarbon-based materials, such as natural gas, coal gas and methanol, into electric energy through an electrochemical reaction.
Unlike a conventional power generation technology requiring a variety of processes, such as combustion of fuel, generation of steam, driving of a turbine, and driving of a power generator, the fuel cells neither require combustion of fuel nor use driving devices, so the fuel cells are advantageous in that the fuel cells can realize high operational efficiency, produce few air pollutants, such as SOx and NOx, reduce the amount of carbon dioxide generated therefrom, and are less likely to produce operational noises or vibrations.
Various kinds of fuel cells have been proposed and used in the related art. For example, phosphoric acid fuel cells (PAFC), alkaline fuel cells (AFC), polymer electrolyte membrane fuel cells (PEMFC), direct methanol fuel cells (DMFC), and solid oxide fuel cells (SOFC) have been proposed and used in the related art.
The solid oxide fuel cell (SOFC) is a fuel cell, in which a solid oxide through which oxygen ions or hydrogen ions can permeate is used as an electrolyte. In the solid oxide fuel cell (SOFC), all the elements constituting the fuel cell are solid elements, so the solid oxide fuel cell is advantageous in that it has a simple construction, is free from loss of the electrolyte, thereby requiring no replenishment of the electrolyte, and is free from corrosion of other materials, compared to other type fuel cells. Further, the solid oxide fuel cell is operated at a high temperature, so the solid oxide fuel cell does not require precious metal catalysts, but fuel can be easily and efficiently supplied to the fuel cell through a direct internal reforming process. Another advantage of the solid oxide fuel cell (SOFC) resides in that the fuel cell discharges high-temperature gas, so the solid oxide fuel cell can be efficiently used for combined heat and power generation using waste heat.
In the solid oxide fuel cell (SOFC), electrode reactions expressed by the following reaction formulas are performed.
<Reaction Formulas>Fuel electrode: H2+O2−→H2O+2e−CO+O2−→CO2+2e−Air electrode: O2+4e−→2O2−Overall reaction: H2+CO+O2→H20+CO2 
In the fuel cell operated according to the above-mentioned reaction formulas, electrons reach the air electrode after passing through an external circuit, and, at the same time, oxygen ions generated from the air electrode move to the fuel electrode via the electrolyte, so hydrogen or CO is combined with the oxygen ions at the fuel electrode, thereby producing electrons and water or CO2.
In a solid oxide fuel cell system, a stack that is formed by laminating a plurality of unit cells is used as a base unit. To increase the capacity of the solid oxide fuel cell system, a plurality of fuel cell stacks is connected to each other in series, in parallel or in series-parallel.
FIG. 1 is a view illustrating the construction of a related art fuel cell system, in which problems of related art fuel cell stacks are shown.
As shown in FIG. 1, the related art fuel cell system 11 includes a power generating unit 19 formed by laminating a plurality of unit stacks 19a, 19b and 19c, a heat exchanger 13, a burner 15, a reformer 17, a power rectifier 27, etc.
Here, the unit stacks 19a, 19b and 19c are combined with each other in a state in which laminated unit cells are enclosed in each unit stack that comes into close contact with neighboring unit stacks. Electric power generated by the power generating unit 19 is appropriately processed by the power rectifier 27, and is then supplied to an external device requiring electric power.
The burner 15 receives gas from the back of the unit stacks 19a, 19b and 19c via a recycling pipe 25, and heats both the reformer 17 and the heat exchanger 13, and causes the reformer 17 to reform fuel, thereby supplying hydrogen-rich gas to the respective unit stacks of the power generating unit 19.
In the heat exchanger 13, heat is transferred from high-temperature unreacted gas (hydrogen, air, etc.) that has been discharged from the power generating unit 19 and recycled by the recycling pipe 25 to fuel and air that have been newly introduced from the outside, thereby heating the fuel and air.
However, the related art fuel cell system 11 is problematic in that, because the unit stacks constituting the power generating unit 19 come into close contact with each other, deterioration in operational performance of a stack may easily ill-affect the other stacks placed near the deteriorated stack. In other words, a reduction in the performance of a stack may be easily propagated to the other stacks.
Described in detail, when a problem occurs in a unit stack and deteriorates the performance of the unit stack, a leaning of current from the deteriorated stack to the other stacks placed near the deteriorated stack is generated, so a heat balance between the unit stacks may be broken, thereby greatly reducing the performance of the fuel cell system 11. In other words, when the temperature of a unit stack is reduced (due to various abnormal reasons), temperatures of neighboring unit stacks that come into close contact with the deteriorated unit stack will be reduced, thereby inducing a great reduction in the performance of the fuel cell system.
As well known to those skilled in the art, in a high-temperature fuel cell, such as SOFC, the operating temperatures of respective unit stacks impose great effect on the operational performance of the stacks, such as output power and durability of the stacks, so it is very important to maintain desired operating temperatures of the unit stacks and to maintain a heat balance between neighboring unit stacks. However, in the conventional power generating unit 19, the unit stacks are brought into close contact with each other, and no heating unit for increasing the temperature of a unit stack to a normal temperature range when the temperature of the unit stack is reduced is provided, so it is difficult to maintain optimal output power of the fuel cell system.